The above-mentioned compound [XVI] useful as an HIV protease inhibitor has been known from WO95/09843, and the production methods thereof have been known from WO97/11937 and WO97/11938.
However, these production methods cannot impart stereoselectivity to intermediates, and thus, they are not satisfactory in view of low yields of the objective intermediates. In addition, these methods require heating to 80-90.degree. C. during reaction, as well as purification due to crystallization, since the resulting intermediate compound is a 1:1 mixture of isomers.